Iso Observations of Fine-Structure Atomic Lines from Proto-Planetary Nebulae

257 ISO OBSERVATIONS OF FINE-STRUCTURE ATOMIC LINES FROM PROTO-PLANETARY NEBULAE A. Castro-Carrizo1,D.Fong2, V. Bujarrabal1, M. Meixner2, A.G.G.M. Tielens3, W.B. Latter4,and M.J. Barlow5 1Observatorio Astronómico Nacional, Apartado 1143, E-28800 Alcalá de Henares, Spain 2University of Illinois, 1002 W. Green St., Urbana, Il.61801, USA 3Kapteyn Astronomical Institute, P.O. Box 800, 9700 AV Groningen, The Netherlands 4SIRTF Science Center/IPAC, CalTech, MS 314-6, Pasadena, CA 91125, USA 5Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, United Kingdom Abstract ejects a very fast and collimated wind that collides with We present ISO observations of fine-structure atomic the remnant of the AGB envelope. This phenomenom is lines from 24 evolved sources. Most of them are proto- thought to determine the morphology and dynamics of the planetary nebulae (PPNe) but we also include a few AGB PPN. stars and planetary nebulae. Data on O0,C+,N+,Si0, Both the increase of the temperature of the central Si+,S0,Fe0 and Fe+ were obtained. PPNe are found star and the presence of shocks can originate the dissoci- to emit through low-excitation atomic lines only when ation of the molecules in the envelope. The photodissoci- the central star is hotter than ∼ 10000 K. This result ation regions (PDRs), that were theoretically studied by suggests that such lines predominantly arise from Photo- Tielens & Hollenbach (1985), are mainly cooled through Dissociation Regions (PDRs). Our results are also in rea- fine-structure atomic lines, the heating being due to the sonable agreement with predictions from PDR emission far-ultraviolet photons absorved by the dust grains. Those lines are excited collisionaly at temperatures of ∼ 102- models, allowing the estimation of the density of the emit- 3 ting layers from comparison with the model parameters. 10 K. On the other hand, the presence of shocks could However, Fabry-Perot ISO observations suggest in some play a role in the formation of relatively wide regions cases a contribution from shocked regions, in spite of their of atomic gas, which could also be cooled through fine- poor sensitivity and spectral resolution. The intensity of structure atomic lines (see Hollenbach & McKee 1989). the [C ii] 158 µm line has been used to measure the amount In this paper we analyze the results of our ISO data of low-excitation atomic mass in PPNe, since this tran- (that in some cases have been improved with data found sition has been found to be a useful model-independent in the ISO data archive or in Liu et al. 2001), and their probe to estimate the total mass of this component. In comparison with models in order to infer the origin of the most evolved sources the atomic mass is very high the emission of the low-excitation atomic gas. In Castro- (up to ∼ 1 M), representing in some cases the dominant Carrizo et al. (2001) and Fong et al. (2001) a detailed nebular component. description of the data, the comparison with models, and the calculation of the masses are found, for O-rich and Key words: Atomic data – Stars: AGB and post-AGB – C-rich sources respectively. (Stars:) circumstellar matter – Stars: mass-loss – (ISM:) planetary nebulae 2. ISO observations Our sample is composed of 24 sources, mostly PPNe but also including a few planetary nebulae and AGB stars for 1. Introduction comparison. The observed O-rich sources are: OH 26.5+0.6, Mira, Betelgeuse, R Sct, AFGL 2343, HD 161796, 89 Her, In the last phases of the life of low mass stars (< 8 M), a M 1–92, M 2–9, Hb 12, Mz–3, and NGC 6302. The C-rich fast evolution from the Asymptotic Giant Branch (AGB) sources are: IRC +10216, IRAS 15194–5115, LP And, to planetary nebulae (PNe) takes place. AGB stars lose IRAS 22272+5435, AC Her, SAO 163075, AFGL 2688, Red most of their mass, forming a cool molecular envelope that Rectangle, IRAS 21282+5050, AFGL 618, NGC 6720, and expands isotropically at low velocity (∼ 15 km s−1). In less NGC 7027. We also observed off-source points to deter- than 1000 years these cool and extended AGB stars, and mine if interstellar medium (ISM) contribution exists, and their respective molecular envelopes, evolve becoming a in that case to estimate the emission that comes from the tiny and very hot blue dwarf surrounded by a mostly ion- nebula. ized nebula. In the intermediate stage, the proto-planetary In order to study the low-excitation atomic gas we nebulae (PPNe) often show intermediate chemical and observed the following fine-structure atomic lines: [O i] physical properties between those of AGB envelopes and (63.2 µm, 145.5 µm), [C ii] (157.7 µm), [N ii] (121.9 µm), [Si i] PNe. At some point of this evolution the post-AGB star (68.5 µm, 129.7 µm), [Si ii] (34.8 µm), [S i] (25.2 µm), [Fe i] Proc. Symposium ‘The Promise of the Herschel Space Observatory’ 12–15 December 2000, Toledo, Spain ESA SP-460, July 2001, eds. G.L. Pilbratt, J. Cernicharo, A.M. Heras, T. Prusti, & R. Harris 258 A. Castro-Carrizo et al. Figure 1. Distribution in the H–R diagram of the observed sources. (24.0 µm, 34.7 µm) and [Fe ii] (26.0 µm, 35.3 µm). We used detected. For a subsequent interpretation of the data it is both ISO spectrometers, LWS (43–196.7µm) and SWS also important to note that the number of detected lines (2.4–45 µm), according to different observational modes, and their intensities increase for sources surrounding hot- grating and Fabry-Perot (FP). ter central star. Only the FP observations allowed obtaining enough We have also found that the ratio of the most intense i ii spectral resolution to study the kinematics of the emitting lines, [O ]63µm/[C ] 158 µm, is very different when emis- sources, but due to the poor sensitivity of the FP modes sion comes from sources or from ISM contamination, being the analysis becomes very difficult. On the other hand, the < 1whenemissioncomesfromISMand> or 1whenit more sensitive grating modes do allow us to get reliable comes from the nebulae. This fact has helped us to infer line fluxes for the detected sources. (All our LWS-FP lines the origin of the detections in those few cases for which have been also observed by grating modes.) The typical we did not observed off-source points. rms obtained with the LWS/SWS grating is ∼ 0.1-1/1-10 −12 −2 −1 −1 10 erg cm s µm . The peak intensity of the most 3. Interpretation of the data intense detected lines is ∼10−8 erg cm−2s−1µm−1 for PNe, and ∼10−9-10−10 erg cm−2s−1µm−1 for PPNe. 3.1. Dependence on stellar parameters In Figure 1 we represent all the observed nebulae in One of the main results of this work is obtained by compar- an H-R diagram, showing the temperatures and the lumi- ison of the observed data with the stellar parameters, i.e., nosities of the central stars. The sources are marked with with the evolutionary stage of the sources. Figure 1 shows filled symbols when some of the observed atomic lines was that only nebulae surrounding stars hotter that ∼ 10000 K ISO Observations of Fine-Structure Atomic Lines from Proto-planetary Nebulae 259 are detected, and we have mentioned that as the Teff in- 3.3. PDR models creases above 10000 K the line emission is more intense and the number of lines detected is higher. The only excep- Our data have been compared with predictions of PDR tions are the detection of several lines in Betelgeuse and models. For the O-rich sources we used the last improved the probable detection [O i]63µm in Mira. Betelgeuse is a version of the models of Tielens & Hollenbach (1985), pub- red supergiant star which shows a well studied UV excess, lished by van den Ancker (1999). For the C-rich sources probably due to a very hot chromosphere, and Mira has new models have been developed by Latter & Tielens a hotter binary companion. Moreover, this dependence on (2001) taking into account the complex and different treat- the stellar temperature is strengthened by the fact that ment of the chemistry, what necessarily changes the phys- among emitters and non-emitters there are very similar ical conditions of the gas. The models solve the chemi- objects, from the point of view of their morphology, pres- cal and the thermal balance in the gas, and predict the ence of shocks, chemistry and total nebular mass, but with emission from these regions primarily as a function of the different low-excitation atomic emission and different Teff incident far-ultraviolet flux (G)andofthedensity(n)in of the central star (see for example the case of AFGL 2688 that region. Note the importance of the parameter G,that and AFGL 618). It is also noticeable the lack of emission represents the radiation field relevant for the photoelectric from the very massive molecular nebulae AFGL 2343 and effect, and which is the main PDR heater. HD 161796. The comparison of our data with models is quite satis- factory, since the physical parameters required to explain We interpret this result as showing that the total mass thedataarequitecompatibleforthedifferentlinesof of the low-excitation atomic gas strongly depends on the each source. In particular, the densities obtained from this temperature of the central star. This suggests that the analysis mostly range from 104 to 106 particles per cm−3.

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